Add pre-commit hook

This pre-commit hook checks the migration files in the current
repository and ensures that they all have a `safe` property
defined. While we can think of easy ways to be break this, for
all the use-cases I've had this covers it quite nicely, so I
think it's worth merging even if we think it's a bit rough.

Tim Schilling wrote this in
#39
and I'm very happy to take that almost verbatim and prove it.
The only thing I changed was to make a new function to handle
sys.exit(), so that I could set it as a console_script to use
with pre-commit.

.gitignore

@@ -10,3 +10,4 @@ db.sqlite3
 htmlcov/
 dist/
 .tox/
+.venv/

.pre-commit-config.yaml

@@ -52,3 +52,7 @@ repos:
     rev: v0.12.1
     hooks:
       - id: validate-pyproject
+-   repo: .
+    rev: HEAD
+    hooks:
+    -   id: check

.pre-commit-hooks.yaml

@@ -0,0 +1,6 @@
+-   id: check
+    name: Mark Migrations for Safety
+    description: Ensure that all local migrations have been marked for safety.
+    entry: safemigrate-check
+    language: python
+    types: [text]

pyproject.toml

@@ -33,3 +33,6 @@ django = ">=3.2,<5.0"
 
 [tool.poetry.dev-dependencies]
 tox = "*"
+
+[tool.poetry.plugins."console_scripts"]
+"safemigrate-check" = "django_safemigrate.check:check"

src/django_safemigrate/check.py

@@ -0,0 +1,69 @@
+"""
+Search our application's migrations are using django_safemigrate.
+
+This is fairly rudimentary and won't work if the class doesn't explicitly inherit
+from ``Migration``.
+"""
+
+import os
+import re
+import sys
+
+MIGRATION_PATTERN = re.compile(r"class\s+(?P<MigrationClass>\w+)\s?\(.*Migration\):")
+MIGRATION_FILE_PATTERN = re.compile(r"^\d{4}_.+\.py$")
+
+MISSING_SAFE_MESSAGE = (
+    "{file_path}: {migration_class} is missing the 'safe' attribute.\n"
+)
+FAILURE_MESSAGE = (
+    "\n"
+    "Add the following to the migration class:\n"
+    "\n"
+    "from django_safemigrate import Safe\n"
+    "class Migration(migrations.Migration):\n"
+    "    safe = Safe.before_deploy\n"
+    "\n"
+    "You can also use the following:\n"
+    "    safe = Safe.always\n"
+    "    safe = Safe.after_deploy\n"
+)
+
+
+def find_migration_files(directory):
+    for root, _, files in os.walk(directory):
+        for file in files:
+            file_is_migration = MIGRATION_FILE_PATTERN.match(file)
+            if file_is_migration and file.endswith(".py"):
+                yield os.path.join(root, file)
+
+
+def validate_migrations():
+    success = True
+    for file_path in find_migration_files("."):
+        # Strip the leading ./ it's a byproduct of os.walk on the current directory
+        file_path = file_path[2:] if file_path.startswith("./") else file_path
+        with open(file_path, "r") as f:
+            content = f.read()
+
+        match = MIGRATION_PATTERN.search(content)
+        if match:
+            migration_class = match.group("MigrationClass")
+            if "safe = Safe." not in content:
+                success = False
+                sys.stdout.write(
+                    MISSING_SAFE_MESSAGE.format(
+                        file_path=file_path, migration_class=migration_class
+                    )
+                )
+    if not success:
+        sys.stdout.write(FAILURE_MESSAGE)
+    return success
+
+
+def check():
+    if not validate_migrations():
+        sys.exit(1)
+
+
+if __name__ == "__main__":
+    check()